In lead-acid batteries, a finely divided mixture of lead oxide powder and metallic lead particles is the key ingredient in preparing positive and negative active material. It is known that the physical and chemical characteristics of this leady litharge, or battery oxide, have a profound impact on the performance and life expectancy of the battery. Battery oxide is manufactured from either primary (mined lead) or secondary (recycled or refined) lead. The lead used from either source must be of extremely high purity; and, commercially, battery oxide is typically made using either the Barton or ball mill process.
In the Barton process, molten lead is metered into a reactor or a Barton Pot where it impinges on a rotating paddle. The resulting droplets of lead are oxidized and can fade out in an air stream.
The ball mill process converts solid lead pieces directly into oxide through attrition by causing them to rub against each other within a rotating reactor or ball mill. The lead in the ball mill is the grinding media as well as a reactant. The particles or agglomerates produced by the molten lead Barton process are droplike or spherical whereas those produced by the solid state ball mill grinding process are flat or flakelike.
In general, the oxidation reaction in either process is controlled to provide the composition desired, typically approximately 75% of the product being oxidized to lead oxide, leaving about 25% as finely divided metallic lead to be oxidized in subsequent processes. In addition to chemical composition, particle size or reactivity of the material is measured and controlled. Among the process control variables there are included reactor temperature, air flow and lead feed rate.
The oxide exiting either the Barton or ball mill reactor is conveyed by an air stream to separating equipment, i.e., a settling tank, cyclone and bag house, after which it is stored in silos, large hoppers or drums for use in paste mixing. Purity of the lead feed stock is extremely critical because minute quantities of some impurities can either accelerate or slow the oxidation reaction markedly.
Still further, recovery from scrap is an important source for the lead demands of the United States and the rest of the world. In the United States, according to some sources, over 70% of the lead requirements are satisfied by recycled lead products. The principal types of scrap are battery plates and paste, drosses, skimmings, and industrial scrap, such as solders, cable sheathing, and the like. Most scrap is a combination of metallic lead and its alloying constituents mixed with compounds of these metals, usually oxides and sulfates. Therefore, recovery as metals requires reduction and refining procedures.
Because it is estimated that some 80% of the lead consumed in the United States is for use in lead-acid batteries, most recycled lead derives from this source of scrap. More than 95% of the lead is reclaimed. Hence, the bulk of the recycling industry is centered on the processing of lead battery scrap.
In general, such a recycling process involves releasing the lead-bearing components from the case and other non-lead containing parts, followed by the smelting of the battery plates, and refinement to pure lead or specification alloys. Each step in the secondary operations must meet the environmental standards for lead in the concentration in air. There are well-defined, sophisticated technologies for the recovery of all materials of value in a battery. Often, after acid removal, scrap batteries are fed to a hammer mill in which they are ground to relatively small particles. The ground components are fed to a conveyor and passed by a magnet to remove undesirable contamination. The lead scrap is then classified on a wet screen through which fine particles of lead sulfate and lead oxide pass, and the large oversized solid particles are passed on to a separator, such as a hydrodynamic separator. The fine particles are settled to a thick slurry and to a clarified wash water recirculated to the wet screen. The paste recovered in the initial screening stage is often not in a form suitable for lead smelting because of its high sulfate content so a desulfurization step is typically carried out. Almost all battery scrap and paste are converted to impure lead or lead alloys by pyrometallurgical processes employing blast, reverberatory, rotary, Isasmelt, or electric furnaces.
The most common procedures for making leady oxide or battery oxide rely on specifying the required purity level of the starting lead before oxidation. The requirements vary depending upon the type of application.
For example, sealed lead-acid cells, often termed "VRLA" cells (viz., valve-regulated lead-acid), are widely used in commerce today. As is known, sealed lead-acid cells utilize highly absorbent separators, and the necessary electrolyte is absorbed in the separators and the plates. Such sealed lead-acid cells and batteries are widely used in commerce today for various applications that have widely differing requirements. In one type of application, generally termed as stationary applications, lead-acid cells and batteries are used, for example, for load leveling, emergency lighting and commercial buildings, as stand-by power for cable television systems, and in uninterruptible power supplies. The uninterruptible power supply may be used to back up electronic equipment, such as, for example, telecommunication and computer systems, and even as a back up energy source for entire manufacturing plants. In addition, there are many applications where sealed lead-acid cells and batteries are used in what are termed motive power applications. Such lead-acid cells and batteries are thus used as a power source for electric vehicles, fork-lift trucks, and the like.
The operation of VRLA cells and batteries is extremely complex and involves a variety of aspects. One important aspect is that VRLA cells must avoid conditions in service in which the temperature within the cell increases uncontrollably and irreversibly. Such a condition is often termed "thermal runaway" in such cells. Thermal runaway is an ongoing issue which is critical in designing such cells and batteries.
For this reason, among others, at least some VRLA cell and battery manufacturers utilize primary lead. The objective is to minimize the level of contaminants as much as possible.
Other types of lead-acid batteries are flooded electrolyte batteries, such as automotive batteries. In this type of application, secondary lead is often used. Nevertheless, it is extremely important that the leady oxide used in such batteries has satisfactory purity levels as regards particular contaminants.
Yet, insofar as the inventors are concerned, there are no commercial methods available for directly lowering the impurity levels, and thereby satisfactorily purifying leady oxides having undue levels of various undesirable contaminants. While some leady oxide manufacturers may occasionally dilute a contaminated oxide by adding a specified amount of an oxide of high purity to effectively lower the total contamination level to an acceptable level, this procedure cannot always be applied to battery grade oxides. More particularly, such a dilution technique can effect previously specified physical properties, such as particle size and surface area.
Still further, none of the other possible procedures that may be envisioned to deal with impure leady oxides are satisfactory. While chemical conversion of the oxide to a suitable lead salt and then utilizing specific gravity concentrators can be considered, a process of this sort is typically a precursor to other refining processes such as pyrometallurgy or electrometallurgy. Alternatively, such impure leady oxides can be converted back to a lead metal utilizing pyrometallurgy (smelting and drossing processes) which, through additions of other elements and the selection of suitable temperatures, may allow removal of some undesired contaminants from the lead. Obviously, the high purity lead produced must then be oxidized to a suitable form of lead oxide for battery use.
As is apparent from the above discussion, both pyrometallurgy and electrometallurgy require several process steps going back to the preparation of the feed lead for the oxidation process. These additional steps can add substantial costs and do not allow any recovery of an oxide batch or lot that has an acceptable level of impurities. Further, electrometallurgy is still used sparingly in practice since its economic justification normally relies on the presence of sufficient contaminating impurities to make the value of the collected impurities cover the additional cost. As an aside, the excessive costs in such processes derive from the need for extensive environmental controls, the need to carefully control the conditions of the aqueous solutions and electrical plating parameters, and the associated plating bath maintenance costs.
As a further and critical complication, pyrometallurgy and electrometallurgy do not address all contaminants. Specifically, two problem contaminants are bismuth and silver, the latter being caused by the increase in use of lead-based alloys including some level of silver as an alloying ingredient. For silver, a complex, multi-step process is used. This process, called the Parkes process, uses excess zinc to precipitate with the silver impurity (also works for gold, copper, tellurium and platinum). After the Parkes process, which in itself has many steps, the lead needs to be de-zinced by either vacuum distillation or oxidation with caustic soda (the Harris process). Bismuth is removed using the Betterton-Kroll process after dezincing. This process forms CaMg.sub.2 Bi as dross that can be removed. Again, the kettle needs to be treated by the Harris process or chlorination to remove the calcium and magnesium.
It is known that leady oxide roasted at a temperature greater than 480.degree. C. and probably, at a temperature near 600.degree. C., for an adequate period of time can convert most of the leady oxide to an orthorhombic(.beta.)PbO form. Further, it has been appreciated that the leady oxide mixture will convert to PbO(.beta.) in such a manner that in excess of 90% of certain impurities will be trapped in the tetragonal(.alpha.)PbO that has not been converted. This research was reported by Boher and Garnier, J. Solid State Chem., 55, pp. 245-248 (1984). In general, the table in that report shows that the tetragonal lead oxide has trapped therein about 90% or more of the manganese, copper, iron, and zinc impurities. Likewise, at this point, the tetragonal and orthorhombic PbO are of similar particle size, with the median particle size being typically somewhere between 4 and 8 microns. Such a mixture can comprise as much as 95% or even more of the orthorhombic species.
Despite the clear need for an effective process for directly purifying leady oxides, no such process exists. It would be highly desirable to have a refined leady oxide product that VRLA battery manufacturers would consider satisfactory and to provide such leady oxides that are adequately free of problem contaminants so that automotive battery manufacturers would not have to incur the expense of costly purification methods.
Accordingly, and in general, a principal object of the present invention is to provide a facile process for directly purifying leady oxides.
Yet another object lies in the provision of a process that can achieve highly purified leady oxides in a cost-effective way relative to the use of electrometallurgy and pyrometallurgy processes.
A still further and more specific object is to provide such a process which is capable of being readily incorporated into existing processes for making leady oxides.
Even more specifically, an object of this invention is to provide a process for purifying leady oxides which can be readily combined with a Barton Pot process.
Yet another object of the present invention is to provide a process capable of at least substantially eliminating the presence of problem contaminants in the purified leady oxides. A more specific object in this connection lies in the provision of a process for purifying leady oxides which can reduce to the level desired such problem contaminants as bismuth and silver.
Other objects and advantages of the present invention will become apparent as the following description proceeds, taken in conjunction with the accompanying drawings. While the present invention is susceptible to various modifications and alternative forms, specific embodiments thereof will be described in detail herein. It should be understood, however, that it is not intended to limit the invention to the particular forms disclosed, but, on the contrary, the intention to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as expressed in the appended claims. Thus, while the present invention will be principally described in connection with purifying leady oxides, it should be appreciated that the present invention can be utilized to provide a soft lead having high purity, whether used in the battery, or in other, fields.